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Serena Williams upset, Venus reaches Cincy semis

Venus Williams, of the United States, leaps after defeating Samantha Stosur, from Australia, 6-2, 6-7 …

MASON, Ohio (AP) — Only one of the Williams sisters is heading to a semifinal, and it's not the one with the long winning streak. Serena Williams lost that — and her cool — at the Western & Southern Open.

Williams had her 19-match streak snapped by Germany's Angelique Kerber on Friday, a 6-4, 6-4 defeat that marked the first time she'd even lost a set since her title at Wimbledon. The third-longest winning streak of her career didn't end quietly.

She flung her racket at the ground, picked it up and slammed it on the court again as the second set slipped away.

"I probably need a break," she said.

While Serena fumed over missed shots, older sister Venus had another jump-for-joy day. She reached the semifinals of a tournament for the first time this season by beating Samantha Stosur 6-2, 6-7 (2), 6-4.

On the men's side, Novak Djokovic and Juan Martin del Potro won in straight sets, setting up a rematch of their bronze medal match at the Olympics. Del Potro won that one.

Serena Williams reacts after missing a point against Angelique Kerber, from Germany, during a quarte …

After all of her summer success on Wimbledon's grass, Serena Williams is having a tough time getting readjusted to hard surfaces. One of the best stretches of her career included single and doubles gold medals at the Olympics in London.

She came into Friday's match with a 64-4 record in the summer hard-court season over the last two years. Against Kerber, her game fell apart.

She missed so many easy shots that after she hit an overhead that landed in the court, she smiled and raised her arms in mock triumph.

Her sister thinks the last few weeks have caught up with her.

"She's played so many matches since Wimbledon," Venus Williams said, as her sister struggled on the court. "I don't think anyone has played as many matches and played as successfully as her.

Serena Williams, of the United States, reacts after missing a point against Angelique Kerber, from G …

"I think some rest is in order for her."

Wearing an Olympics shirt after her match, Serena Williams agreed she needs a little time to get recharged for the U.S. Open.

"I'll be better in New York, I promise," she said.

First, she's going to stick around to watch her sister's resurgence. Venus Williams raised both arms and jumped — kicking up her heels — after winning a three-setter for the third time in the tournament, an indication she's learning to manage an immune system disorder that leaves her tired.

Novak Djokovic, from Serbia, hits a backhand shot against Marin Cilic, from Croatia, during a quarte …

"The good thing: I'm not getting worse, so I can play," he said. "But anyway, I want to wait (for) what the doctor says because I have experience on my other wrist."

Djokovic reached a Masters semifinal for the second straight week by knocking off Marin Cilic 6-3, 6-2. He won the Rogers Cup in Toronto last Sunday.

The Serb was fresh on Friday, having spent only a half-hour on court the previous day. Russia's Nikolay Davydenko had to quit after losing the first set 6-0 on Thursday night because of a sore shoulder that interfered with his serve.

"It was my best match so far here in this tournament," Djokovic said. "Came in the right moment, really."

Federer skipped the Rogers Cup, taking a week off the tour after playing the Olympics. He's been sharp on the hard courts this week, playing what he called "high-quality tennis."

"I thought I played really well tonight," said Federer, who never faced a break point. "Mardy was able to lift his game in the second set."

The women's bracket lost its top seed Friday night. Poland's Agnieszka Radwanska got treatment on her shoulder while losing 6-1, 6-1 to Li, who had to play two matches Friday. Li beat Johanna Larsson 6-2, 6-2 to open the day, a match that had been postponed by rain the previous night.

Petra Kvitova of the Czech Republic beat Russia's Anastasia Pavlyuchenkova 6-3, 7-6 (4) in the last match Friday. Kvitova will play Kerber in the semifinals.